1. Field of the Invention
The present invention relates to an ignition system for internal combustion engines, and is more particularly concerned with an ignition system for internal combustion engines which ensures ignition.
2. Discussion of Background
Referring to FIG. 3, there is shown a schematic circuit diagram of a conventional ignition system for internal combustion engines, which has been disclosed in e.g. Japanese Unexamined Patent Publication No. 148071/1989. In FIG. 3, reference numeral 1 designates a power circuit. The power circuit is constituted by a battery 2 and a converter. The converter includes a transformer 3 connected to the battery 2, a transistor 4 connected to a transformer 3, a transistor 5 connected to the base of the transistor 4, a diode 7 connected to the base of the transistor 5, and an oscillation circuit 8 connected to the diode 7. The transformer 3 has the secondary winding connected to the primary winding of an ignition coil 11 through a diode 9 and a capacitor 10. The ignition coil 11 has the secondary winding connected to a spark plug 12. The capacitor 10 has one end grounded through a diode 13. The capacitor 10 has the other end grounded through a discharge control thyristor 14. The thyristor 14 has the gate connected to a signal coil 16 through a spark-timing control circuit 15. Reference numeral 17 designates a power supply prevention circuit which includes a comparator 18. The comparator has a noninverted terminal connected to the gate of the thyristor 14. The comparator has an inverted terminal connected to the junction between reference resistors 19 and 20. The comparator has an output terminal connected to the base of the transistor 5 through a diode 22. Reference number 21 designates a resistor which is connected to the output terminal of the comparator 18.
The operation of the conventional ignition system shown in FIG. 3 will be described. The voltage from the battery 2 is increased to a high level by the transformer 3. The high voltage is rectified by the diode 9, and charges a capacitor 10. A signal which is generated from the signal coil 16 in synchronism with engine speed is supplied to the gate of the thyristor 14 through the spark-timing control circuit 15, thereby causing the thyristor 14 to conduct. As a result, the electric charges stored in the capacitor 10 are discharged through the thyristor 14 and the primary winding of the ignition coil 11 to cause a high voltage to generate at the secondary winding of the ignition coil 11, thereby firing the spark plug 12.
The voltage across the gate and the cathode of the thyristor 14 which is indicated by V.sub.g at FIG. 4(a) is supplied to the comparator 18 to be compared to a set voltage V.sub.r. When the voltage V.sub.g across the gate and the cathode achieves the set voltage V.sub.r or more, an output shown in FIG. 4(b) is obtained at the output of the comparator 18 to turn on the transistor 5. As a result, the transistor 4 is turned off to obtain the collector voltage in the form shown in FIG. 4(c), thereby substantially forcing oscillation to stop. In this manner, the output from the power circuit 1 is prevented from being supplied to the capacitor 10.
As stated earlier, the conventional ignition system detects the voltage across the gate and the cathode of the thyristor 14 before stopping oscillation. This arrangement creates a problem in that the magnetic energy which has been stored in the transformer 3 is discharged to the thyristor 14 just before oscillation stoppage, and the thyristor 14 is kept conducting for a long period to shorten the subsequent oscillation charging period, causing the capacitor 10 to be charged in an incomplete manner.
The effect which is given by the continuous conduction of the thyristor 14 for such long period grows great in particular at high engine speed because the cycle from one spark from the following spark is shortened. This creates another problem in that the thyristor 14 could be ultimately kept conducting until the following spark-timing, causing misfire.
In addition, because oscillation starts immediately when the voltage across the gate and the cathode of the thyristor 14 lowers, a voltage could be applied to the thyristor 14 by the power circuit 1 before the withstand voltage of the thyristor 14 has fully revived. This means that there is a possibility that the thyristor 14 is conducted again to short-circuit the output of the power circuit 1, creating another problem in that the capacitor 10 is charged in an incomplete manner.